The present invention relates to prestressing steel materials for use with concrete that is prestressed by posttensioning. In particular, the present invention relates to a prestressing steel materials subjected to the posttensioning to be in an unbonded state in which the steel material is not bonded to the concrete.
Concrete has a relatively low tensile strength. In order to overcome this disadvantage, prestressed concrete has been developed. By means of high strength steel wires, bars or strands, a concrete member is precompressed. When the structure receives a load, the compression is relieved on that portion which would normally be in tension.
There are two general methods of prestressing, namely, pretensioning and posttensioning. The present invention relates to prestressing steel materials for use with concrete of the type that is prestressed by posttensioning.
Structural designs used to prevent direct contact between prestressing steel materials and the surrounding prestressed concrete are illustrated in FIGS. 1 and 2. The design shown in FIG. 1 can be used whether the steel material is in the form of a wire, bar or strand. A steel member 1 having a grease coating 2 is sheathed with a PE (polyethylene) tube 3. When the steel member 1 with the PE tube 3 is placed within a concrete section 4, the lubricating effect of the intermediate grease coating 2 reduces the coefficient of friction between the steel member and concrete to as low as between 0.002 and 0.005 m.sup.-1. Because of this low coefficient of friction, the design in FIG. 1 provides great ease in posttensioning a long steel cable in concrete. However, if the steel material is of short length, the need for preventing grease leakage from either end of the PE tube presents great difficulty in fabricating and handling the steel material. Furthermore, steel members having screws or heads at both ends are difficult to produce in a continuous fashion.
The steel member 1 shown in FIG. 2, which is encapsulated in asphalt 5 and embedded in a concrete section 4, has a slightly greater coefficient of friction than the structure shown in FIG. 1. This design is extensively used with relatively short prestressing steel materials since it is simple in construction, is leak-free, and provides ease in unbonding the steel material from the concrete, even if the steel member has screws or heads at end portions.
One problem with the design in FIG. 2 is that the presence of the asphalt (or, alternatively, a paint) may adversely affect the working environment due to the inclusion therein of a volatile organic solvent. Moreover, the floor may be fouled by the splashing of the asphalt or paint. As another problem, great difficulty is involved in handling the coated steel material during drying or positioning within a framework, and separation of the asphalt coating can easily occur unless utmost care is taken in ensuring the desired coating thickness.